Pipelines for transporting fluids, whether they be single-phase or multiphase fluids, can be subject to obstructions. Multiphase fluid refers to the combination of various fluids immiscible with each other, called “phases”, that can be solid, liquid or gaseous and can be more or less closely mixed to form dispersions, emulsions or foams. Obstruction, on the other hand, means a reduction in the diameter of a certain pipeline, typically due to the progressive fouling of the internal walls of the pipeline itself.
An obstruction can be due, for example, to calcareous encrustations or deposits formed by the accumulation of particles suspended in the fluid in the transporting phase. Other factors which influence the formation of obstructions can be associated with the temperature of the fluid in the transporting phase, specific physico-chemical properties of the fluid itself, but also the geometry of the pipeline (for example the presence of sharp bends, bottlenecks, valves, etc.). The obstruction of a pipeline generally causes a pressure drop for the fluid being transported and, in more serious cases, it can also lead to swelling and explosion of the pipeline itself.
Methods and systems have therefore been conceived for detecting obstructions in pipelines for transporting fluids. One of these methods consists in measuring the pressure drops that take place in each section of a certain pipeline. The data measured are then compared with the data calculated in relation to the contemporaneous flow-rate of the fluid in that certain pipeline section. This method can be defined as being “indirect” as it reveals the restriction through its fluid-dynamic effects, in contrast with “direct” detection methods of restrictions in the internal diameter.
The advantages of this indirect measurement method of pressure drops consist in the fact that it is not invasive and can be applied with continuity under both single-phase and multiphase fluid conditions. In addition, this method can be applied under both stationary and transitory conditions. The most evident disadvantage, however, lies in the fact that this method requires a knowledge of the flow-rates in each section of the pipeline, including the intermediate sections, which, in practice, is normally impossible in pipeline networks, where only flow-rates relating to the end junctions are known or estimated.
Another indirect method that can be used for identifying obstructions in pipelines is the passive noise measurement method. This method consists in measuring particular sound emissions characteristic of certain phenomena, such as the movement of fluids through obstructed sections or through specific components of the transportation plant, such as, for example, regulation valves.
The passive noise measurement method is non-invasive and can be applied in continuity, but can only be used under single-phase fluid conditions (gas or liquid). Furthermore, this method requires the use of suitable transducers and a suitable positioning of the same transducers along the pipeline. The main disadvantage of the passive noise measurement method, however, lies in the limitation of the distance between the position of the transducers and that of the obstructions to be revealed. This distance, limited to a few hundreds of meters, is, in fact, strictly linked to the maximum propagation distance of the sound waves inside the fluid.
A further indirect method that can be used for identifying obstructions in pipelines is the method based on pressure waves, or the “back-pressure method”. In other words, this method is based on the generation of a pressure or overpressure wave in the fluid transported inside the pipeline. This overpressure wave travels along the pipeline at a much higher velocity than that of the fluid. The interaction of this wave with possible obstructions present along the pipeline generates overpressure echoes. This method is therefore based on the reception and interpretation of these pressure echoes in order to evaluate the entity of possible obstructions present along the pipeline.
The method based on pressure waves is also non-invasive and also allows restrictions and/or obstructions in very long pipelines, i.e. in the order of hundreds of kilometers, to be detected and localized. This method, however, necessarily requires the use of specific pressure-wave generation equipment, it can only be applied under generically single-phase fluid conditions (gas or liquid), it requires the use of suitable transducers and must be applied separately to each pipeline section of the network.
Yet another method, this time “direct”, that can be used for identifying obstructions in pipelines is the so-called “pigging” method. This operates through launching devices known as “pigs” into the pipelines. “Pigs” are particular shaped probes, frequently pushed by the fluid travelling in the pipeline. In addition to removing liquids/sludge/deposits from the pipeline, “pigs” can also effect measurements on the pipeline itself, possibly detecting and localizing restrictions and/or obstructions.
With the “pigging” method, it is possible to effect extremely accurate measurements also in very long pipelines, i.e. in the order of hundreds of kilometers, but the main disadvantage of this method lies in the fact that it is particularly invasive and has the residual risk of causing transportation problems. This method also requires the use of launch and recovery devices of the “pigs” on each section of the pipeline to be inspected: these devices are often unavailable and/or cannot be applied in all sections of the network.
Finally, it should be noted that none of the traditional methods mentioned above for identifying obstructions in pipelines can be conveniently applied to complex pipeline networks. The above methods, on the contrary, are only satisfactory when applied to pipelines free of intermediate inlet or outlet points of the fluids.